IAP-24-122

Geomechanics of salt cavern operation for underground hydrogen storage

Project aim:

Investigate the mechanical behaviour of rock salt deposits with application to salt cavern operation for low-carbon energy storage (LCES) to understand how far we can safely and economically push the boundaries of cavern creation and operation in terms of cavern depth, injection and withdrawal rates, and operational pressures.

This project will conduct laboratory experiments on rock salt during cyclic injection and withdrawal that will provide valuable evidence about the mechanical behaviour of rock salt relevant for underground hydrogen storage (UHS) and inform larger-scale cavern creation and operation in the rock salt formations at the Boulby Mine Underground Laboratory (STFC-funded BGS-led SUCcESS project).

Background:

Affordable, efficient grid-scale LCES is essential for decarbonisation to meet Net-Zero commitments (IPCC, 2023). LCES technologies enable the storage of excess energy generated from renewable sources during peak periods of production and provides energy during high demand, creating a sizable buffer for a balanced, dependable energy grid. Amongst them, hydrogen storage offers the terawatt hour scale capacity that promises to decouple renewable energy generation from demand, enabling the decarbonisation of difficult to abate sectors such as heating and heavy-duty transport while balancing inter-seasonal energy supply and demand (IEA, 2023).

Gas storage in salt caverns is a proven technology, already implemented in the UK, that can meet these storage needs at the necessary scale (UKGov, 2021). Hydrogen storage in salt caverns in Teesside, first created in 1971, have been operational for about 50 years (Crotogino et al., 2018; Forbes Inskip and Ougier-Simonin, 2021). Salt caverns are attractive for hydrogen storage because rock salt has physical properties of tight fabric and low permeability, as well as mechanical properties of low strength and ductility (e.g. Forbes Inskip and Ougier-Simonin, 2021 and references therein). This ability to creep allows the rock salt to repair stress-induced fractures over time. However, it also means that if the pressure in the cavern is less than the pressure exerted by the surrounding rock, the cavern will close over time. Since closure rates increase as cavern temperature and pressure increase, cavern depth is a key controlling factor on the behaviour of the rock salt, and therefore cavern stability. Shallow caverns are susceptible to chimneying (formation of vertical fractures) in the cavern roof due to lack of strength from the geology above, potentially leading to roof collapse and surface subsidence. In addition, they may not provide economically viable storage capacity due to the reduced pressure exerted by the surrounding rock. As a result, cavern depths of 600-1700 m are considered optimal, with practical limits of 200-2000 m (Warren, 2016; Williams et al., 2022).

UK industrial clusters such as Humber and Teesside aim to store hydrogen in salt caverns to reduce greenhouse gas emissions (Inovyn, 2022; ECH2, 2023). Salt caverns in this region are hosted in the Permian bedded rock salt deposits (part of the Zechstein Group) of the Fordon Evaporite Formation in East Yorkshire at Aldbrough and Hornsea, and the Boulby Halite Formation in Teesside at Wilton and Salthome (Evans, 2008; Williams et al., 2022; Daniels et al., 2023). The Fordon is situated at >1600 m depth and is in places c. 300 m thick, while the Boulby is utilised for gas storage at 350-650 m depth and is c. 30 m thick (Williams et al., 2022; IDRIC 2024).

Research focus:

Balancing daily and seasonal energy demand requires multiple cycle frequency of subsurface hydrogen injection and withdrawal, and the mechanical behaviour of salt caverns over such cycles remains uncertain. Key issues include the mechanical response of heterogeneous reservoir rocks at in-situ reservoir stress and temperature conditions, the potential for fault reactivation and induced seismicity in areas surrounding the caverns, and the memory effect of salt cavern reservoirs during continuous operation (Ramesh Kumar et al., 2023).

Cavern stresses are affected by injection/production flow rates and the temperature of the injected fluids (Forbes Inskip and Ougier-Simonin, 2021). These drive the mechanical and thermal fluctuations that increase the potential for brittle micro-fracturing and spalling. Higher flow rates lead to larger thermal fluctuations that occur over timescale that create temperature gradients between the injected gas and the cavern because the surrounding rock does not have time to adjust to the temperature change. Even if cavern stresses are maintained within geomechanical limits, multiple cycles of injection/withdrawal may lead to fatigue, reducing rock salt strength, inhibiting repair of microfractures and compromising cavern integrity (Ramesh Kumar et al., 2023). During operational hydrogen storage in a salt cavern, the process of injecting and withdrawing hydrogen subjects the cavern to cyclic loading and unloading. For example, during withdrawal, over a timeframe of one week, the fluid pressure is reduced from 80% of lithostatic pressure down to 30%. And vice-versa during injection. Such rapid stress changes are likely to result in significant deformation of the rock salt, which must be carefully managed to avoid structural integrity issues.

Key research questions:

• How does rock salt behave within existing operational boundaries?
• How much can we push these operational boundaries?

Hypothesis:

Multiple cycles of injection and withdrawal induce a memory effect in rock salt due to local stress changes, which influences salt strength and brittle vs ductile deformation mechanisms.

Objectives:

• Characterise the composition and microstructure of samples from the Boulby Halite Formation collected from Boulby mine, and another evaporite formation more representative of deeper thicker formations such as the Fordon Evaporite Formation.
• Conduct core-scale rock deformation experiments at BGS Keyworth to characterise mechanical and micro-seismic behaviour of rock salt under a range of pressure and temperature conditions and injection/withdrawal rates relevant for hydrogen storage in caverns.
• Conduct micro-scale experiments at Heriot-Watt / University of Edinburgh informed by core-scale experiments to optimise experimental protocol for targeted in-situ x-ray imaging experiments at a synchrotron.
• Perform targeted synchrotron experiments to observe microstructural changes directly with x-rays while monitoring mechanical behaviour and micro-seismic activity.
• Analyse the integrated mechanical, micro-seismic and x-ray datasets to visualise microstructural changes, test the hypothesis and address the research questions.
• Undertake geomechanical modelling using conventional statistical methods and artificial intelligence (AI) / machine learning (ML) methods to generate experimentally validated digital twins from the integrated datasets.

This project will conduct experiments to constrain geomechanical controls and limitations on salt cavern operation for underground hydrogen storage.

The first stage of the project will be a thorough literature review, and fieldwork to collect material for characterisation and experimentation. Fieldwork will involve a visit to the Boulby mine to obtain material from the Boulby Halite Formation, and to BGS Keyworth to obtain material from another evaporite formation more representative of the deeper thicker Fordon Evaporite Formation. Microscopy techniques, including SEM, x-ray tomography, ultrasonic velocities and XRF, will be used to characterise composition (percentage evaporite, insolubles) and microstructural textures in samples prepared for experimentation.

Experimental campaigns will involve triaxial rock deformation and fluid injection/withdrawal approaches with integrated mechanical and micro-seismic monitoring. We will use helium as the injected fluid instead of hydrogen as it shares similar properties whilst also being inflammable. Experiments will be conducted at two different scales. The first will be conventional core-scale (Martin-Clave et al., 2021) to characterise how the selected lithologies behave at a range of relevant lithostatic pressures and temperatures ranging from 350 m to 2000 m equivalent depth and then looking deeper than 2000 m, under injection and withdrawal rates equivalent to operational cavern rates of one week and faster. Helium pressures will range from 5% to 95% of lithostatic pressure, healing periods will be integrated into the cycles and samples will be brought to temperature in an oven prior to deformation. These experiments will inform targeted micro-scale experiments to observe directly changes in the microstructure with time-resolved in-situ x-ray imaging at a synchrotron, using novel x-ray transparent triaxial deformation apparatus (Cartwright-Taylor et al., 2022; Freitas et al., 2023) that allow for deformation at elevated pressure with either micro-seismic monitoring or elevated temperature, with protocols optimised in-house prior to beamtime.

Analysis of these integrated experimental datasets will include advanced image processing methods for microstructural analysis, comprising image segmentation to separate cracks and pores from solid phases, and digital volume correlation to obtain local strain field evolution. In addition, acoustic signal processing will be performed, and micro-seismic event history, strain evolution and changes in microstructural geometry and spatial distribution will be integrated with stress history test the hypothesis and address the research questions. Pore-scale modelling using geomechanical and AI/ML regression methods will be undertaken to support this analysis. In addition to peer-reviewed publications, the project will leave a legacy of unique datasets that will be published open-access.

The engineering consultancy AtkinsRéalis will be the project CASE Partner, with Carla Martin-Clave providing co-supervision and hosting a three-month internship within their Energy Storage team towards the end of the first year of the PhD. During the internship, the student will be involved in consultancy projects focussed on assessing the geomechanical integrity of salt caverns in the UK, e.g., the Aldbrough Hydrogen Pathfinder project.

ICL Boulby, the operator of Boulby mine will be the project End User Collaborator, providing specialist knowledge about the Boulby Halite Formation, access to Boulby mine and core store, and facilitating material collection from the mine.

External collaborators include Dr Ian Butler, Prof Katriona Edlmann and Dr Mike Chandler at University of Edinburgh, and Prof. Florian Fusseis at RWTH Aachen University. Drs. Butler and Fusseis will contribute expertise, equipment and facilities to support the targeted synchrotron experimental campaign. Dr Edlmann will contribute expertise on the hydrogen economy in general and underground hydrogen storage specifically. Dr Chandler will contribute expertise in rock salt deformation and geomechanical modelling. Furthermore, Drs Butler and Fusseis are involved in the EXCITE2 network (https://fast.geo.uu.nl/app/public-call/47d3-excite2-call-2) which, in a collaboration between University of Edinburgh and RWTH Aachen University provides funded access to European synchrotron facilities, advanced x-ray transparent rock deformation apparatus and image analysis workflows.

Year 1

Training
· Systematic literature review methods and time management (Heriot-Watt)
· Sample preparation methods, and sample characterisation techniques involving acquisition, processing and analysis for x-ray tomography, ultrasonic velocity, SEM and XRF data (Heriot-Watt / BGS)
· Other Heriot-Watt training sessions for PhD students
· International Summer School on Underground Hydrogen Storage, Europe

Literature review
· Literature review on the geomechanics of hydrogen storage in salt caverns

Sample collection, preparation, characterisation
· Fieldwork to collect material to sample
· Prepare samples for testing
· Perform x-ray tomography, SEM, XRF and run analysis to characterise samples (Heriot-Watt)

Internship
• Three-month internship at AtkinsRéalis at the end of the first year

Communication
· One national conference – Solution Mining Research Institute conference, Spring 2026, Edinburgh. (https://www.solutionmining.org/)
· One review paper

Year 2

Training
· High pressure, high temperature experimental testing (Heriot-Watt / BGS)
· Synchrotron beamline safety and processes (Heriot-Watt / EXCITE2)
· Project management and proposal writing (BGS)
· Heriot-Watt training sessions for PhD students
· ALERT Geomaterials Doctoral School and Workshop, Europe

Experimental work planned
· Cyclic loading experiments (BGS) on samples representative of selected underground storage conditions in the UK. The experimental conditions will be designed to simulate in-situ reservoir environments (saturation, pressure, temperature) and facilitate knowledge transfer from the laboratory- to the field-scale.
· Synchrotron proposal and subsequent campaign for targeted cyclic injection experiments with time-resolved in-situ x-ray imaging to visualise changes in the microstructure and benchmark previously inferred geomechanical behaviour (Heriot-Watt / University of Edinburgh / EXCITE2).

Communication
· One international conference, e.g., EGU (academia-led) or EAGE (industry-led)

Year 3

Training
· Software for Practical Analysis of Materials (SPAM) workshop for Digital Volume Correlation (https://www.spam-project.dev/workshops/), Europe
· Image segmentation and object geometry analysis methods (Heriot-Watt / EXCITE2)
· Geomechanical and AI/ML modelling methods (Heriot-Watt / University of Edinburgh)
· Heriot-Watt training sessions for PhD students

Data integration and evaluation
· Data management and processing
• Data analysis to characterise rock salt mechanical, micro-seismic and microstructural behaviour under in-situ conditions relevant for hydrogen storage in salt caverns
· Development and validation of the pore-scale geomechanical model / digital twin
· Hypothesis testing
· Reproducibility experiments (Heriot-Watt / BGS)

Communication
· One international conference, e.g., EGU (academia-led) or EAGE (industry-led)
· One experimental data paper

Year 3.5

Communication
· Thesis completion
· One hypothesis-testing paper including modelling results

In addition to the IAPETUS training, a comprehensive training programme will be provided comprising both specialist scientific training and generic transferable and professional skills.

Key project-specific training will be given in experimental techniques, managing and analysing mechanical, micro-seismic and large x-ray datasets, and methods for geomechanical and AI/ML modelling. This will be provided by the supervisory team and technical staff at HWU and BGS and external collaborators where appropriate. This specialist training will be complemented by attendance at external workshops and conferences. Attendance at two to three doctoral training schools and two to three international conferences over the course of the project will be encouraged. This attendance will supplement project-specific training, provide exposure to a broad scientific network and audience, and enabling the application of transferable and professional skills developed through relevant University-level and School courses. During the project, the supervisory team will reach out to salt cavern operators in Teesside (Sabic) and Humber (SSE) to identify opportunities for the student to visit a working salt cavern.

The Research Futures Academy (RFA) at Heriot-Watt provides transferable skills / career development workshops to facilitate the doctorate and future research career of PhD students. The student will be able to attend workshops focusing on:
• basic skills for successful research
• research data management
• communication and dissemination skills
• strategy for publishing
• citation and impact
In addition to supporting doctoral students to develop such competencies, the RFA provides valuable networking and collaboration opportunities between researchers across diverse disciplines. Heriot-Watt subscribes to LinkedIn Learning which provides courses in a variety of common software packages, coding languages and other transferable skills that doctoral students have access to for online learning at their own pace. At BGS the student will attend training sessions on scientific paper and proposal writing and project management.

The student will be part of the GeoEnergy and DigiPorFlow research groups at Heriot Watt University working alongside other world-leading researchers in the Lyell Centre for Earth and Marine Science, Institute for Geoenergy Engineering and Institute for Net Zero and Beyond across a range of geo-energy related projects. They will be based in the Lyell Centre and the School for Energy, Geoscience, Infrastructure and Society (EGIS), both of which host annual PhD symposia for students to showcase their work, practice their communication skills and discuss their research topic with a broad audience. They will have access to the substantial computing resources of the 4D Imaging Group and the GeoChemFoam advanced pore-scale modelling project. Alongside this, the student will have access to the BGS Lyell Centre to engage with the Energy Storage research group, where the student will become accustomed to the BGS corporate environment and workflows. The CASE Partner internship with AtkinsRéalis will provide training in professional consultancy and further exposure to professional workflows and working practices. Through the external collaborators, the student will also have access to research networks within University of Edinburgh and RWTH Aachen.

At Heriot-Watt, the student will belong a diverse, inclusive and international community with a strong research culture that celebrates effort and achievement. As part of the IAPETUS DTP cohort, the student will receive a multidisciplinary package of training focused around meeting the specific needs and requirements of each student, benefitting from the combined strength and expertise that is available across the partner organisations.